Implanted cardiac stimulation systems may be used to deliver cardiac resynchronization therapy (CRT) or to otherwise pace the heart. When the heart is paced in the left ventricle (LV), for example, there may be unwanted stimulation of the phrenic nerve that causes contraction of the diaphragm. Unintended phrenic nerve activation (an unintended action potential propagated in the phrenic nerve) is a well-known consequence of left ventricular pacing. The left phrenic nerve, for example, descends on the pericardium to innervate the left part of the diaphragm. In most people, the left phrenic nerve runs close to coronary vein targets for lead implantation. The unintended phrenic nerve activation may cause the diaphragm to undesirably contract. Unintended phrenic nerve activation may feel like hiccups to the patient. Such unintended phrenic nerve activation can occur when the electric field of the LV pacing lead is proximate to the left phrenic nerve and is at a stimulation output that is strong enough to capture the nerve. The unintended phrenic nerve activation may also be referred to herein as pace-induced phrenic nerve stimulation and abbreviated as “PS”.
Unintended phrenic nerve activation may vary from patient to patient. One reason for this variance is that the anatomic location of the phrenic nerve can vary within patients. Additionally, the veins are not always in the same location with respect to the ventricle and the nearby passing nerve. Also, the selected vein in which to place a cardiac lead or the location of the pacing electrode for a prescribed cardiac therapy may vary.
Cardiac therapies may be delivered using different pacing configurations and different stimulation parameters. Examples of pacing configurations include LV bipolar, LV to can, and LV to RV (right ventricle) also referred to as “extended bipolar.” Examples of stimulation parameters include the amplitude (e.g. voltage) and pulse width. The pacing configuration or the stimulation parameters of a therapy may be modified in an effort to avoid phrenic nerve stimulation. The LV pacing electrodes may be repositioned to capture the LV for a pacing therapy such as CRT while avoiding phrenic nerve capture, or the clinician may decide not to implant an LV pacing electrode but rather rely on other pacing algorithms that do not pace the LV.
Although phrenic nerve stimulation is commonly assessed at implant, unintended phrenic nerve activation caused by phrenic nerve capture during pacing may first appear or worsen post-implant for a variety of reasons such as reverse remodeling of the heart, lead micro-dislodgement, changes in posture, and the like. Therefore, the device may be reprogrammed during special office visits after implant to avoid phrenic nerve stimulation. Electronic repositioning (i.e. changing the pacing configuration) is usually effective, but the vector and stimulation parameter reprogramming can be time consuming. Additionally, PS may be present in all vectors. Knowledge of the PS threshold for each available pacing vector/configuration, along with myocardial capture thresholds, may be used to select a viable or optimal stimulation configuration (e.g. LV pacing configuration) that eliminates or reduces the risk for PS.
It is desirable to provide a more effective way to choose the vector with the biggest safety margin (e.g. largest voltage amplitude different between the thresholds of myocardial stimulation and PS). The amplitude of the PS response is dependent on the pacing output. If an accelerometer is used to sense PS, an amplitude of an accelerometer signal for the PS response can be faint and difficult to detect reliably when the pacing energy level is reduced, especially in the presence of noise. A result may be underestimating the PS onset threshold by estimating the PS onset to be higher than the true onset.